Conventional belts for power transmission such as toothed belts, V belts, and V-ribbed belts are commonly made of chloroprene rubber due to its excellent heat, oil and ozone resistance, as well as its other desirable mechanical properties. Chloroprene rubber compositions, particularly the sulfur-denatured types, typically include a metal oxide vulcanizing agent such as zinc oxide or magnesium oxide or a metal oxide with a vulcanization accelerator. Chloroprene rubber compositions of the nonsulfur-denatured type often include both a metal oxide and a vulcanization accelerator.
Chloroprene rubber compositions are often used in power transmission belts because they are less tacky on pulley surfaces than other rubber compositions. Often the chloroprene rubber compositions include sulfur to increase the cross linking density of the rubber and carbon black to increase the hardness of the rubber.
However, with chloroprene rubber compositions of the sulfur-denatured type, the addition of sulfur to this rubber composition results in only a slight increase in the cross linking density of the rubber and detrimentally reduces the heat resistance of the rubber.
Additionally, when large amounts of carbon black are added to increase the hardness of the rubber composition, it becomes difficult or impossible to accurately mold the belt tooth parts, especially by the conventional method wherein the rubber is pressed into a mold. Also, when short fibers and carbon block are added together in high concentrations to increase the strength of the rubber, the workability of the rubber becomes poor, it is difficult to develop the desired belt fatigue resistance and tackiness, and heat and bending resistance diminish.
A power transmission belt including a composition of chloroprene rubber, metal oxide and bismaleimide, without short reinforcing fibers, is disclosed in Japanese Patent Publication No. 46698/1986. This belt has an elevated cross linking density and does not cure prematurely.
Japanese Patent Publication No. 63656/1993 discloses a transmission belt made up of chloroprene rubber, a metal oxide, bismaleimide, and aramid short fibers.
With only metal oxide and bismaleimide added to the chloroprene rubber, the cross linking density of the rubber does not rise sufficiently. This composition, when used in friction driving belts, such as toothed belts, V belts, V-ribbed belts, and the like, generally does not result in a good quality belt.
When a toothed belt is required to transmit a heavy load, insufficient hardness of the teeth can cause cracking and tooth loss to occur. When the teeth are subjected to large compressive forces from cooperating pulleys, and are placed under conditions wherein they are subjected to repeated deformation, the tooth root parts are quickly fatigued and cracking and tooth loss can occur. This loss of teeth from the belt impairs the meshing of the teeth and cooperating pulleys, and may cause the belt to jump off the pulleys.
Additionally, with V belts and V-ribbed belts, even with the addition of large amounts of carbon black and short fibers, it has not been possible to adequately improve belt fatigue resistance and belt tackiness.